Question

A charged particle enters a uniform magnetic field, directed perpendicular to its velocity. The magnetic field may

• A. increase the kinetic energy of the particle.
• B. decrease the kinetic energy of the particle.
• C. change the direction of motion of the particle.
• D. Both options $\left(A\right)$ and $\left(C\right)$

Answer 1

The correct option is C change the direction of motion of the particle.

Force on a charged particle $q$ moving with velocity $\overrightarrow{v}$ in a uniform magnetic field is given by,

$\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})$

$\Rightarrow \overrightarrow{F}\perp \overrightarrow{v}$ all the time and therefore, magnetic force can change direction of motion of charged particle.

$\Rightarrow \overrightarrow{F}\perp \overrightarrow{dr}\left[\overrightarrow{dr}\text{is displacement}\right]$

$\Rightarrow$ No work is done by the magnetic force on the charged particle as work done, $W=\overrightarrow{F}.\overrightarrow{dr}$.

Now, according to work energy theorem, the change in kinetic energy of the charged particle will be zero.

Hence, option $\left(C\right)$ is the correct answer.